I think those of you that are NOT getting errors but did not calibrate the sensor might be fooling yourselves. Based on posts, it seems the system requires us to tell it what the pressure the tire is when filled all the way and zero pressure. If you do not do this, it might assume the current pressure is the max pressure (or does it not notice a new sensor has been installed - the car's computer thinks the prior tires are installed?).
Let's assume your winter tire is 20% low when we install it and we leave it that way. Since the car has nothing else to go by, it might assume the tire is filled 100%. If it falls 10% (from already being underinflated by 20%), the "dummy light" won't come on because the car thinks the tire is only 10% low.
The sensor needs to be either an absolute pressure sensor AND be told what the correct pressure is for the tires currently in use, OR we need to tell it (calibration) what the correct pressure is for the tires. Otherwise there's no way the car can know what pressure is "low" for your tires. Since (according to various posts) we need to calibrate them, it seems to me the sensor is likely not absolute.
I have barely done anything with my car yet. i bought it recently and am driving a winter rat (my 99 Contour with V6)). Couldn't pass up $4500 in incentives, $500 coupon/cash form ford, and the dealer knocked off $1640. $6640 total off. A plain SE sedan manual coudl have been purchased for $13k I think. I paid under $16k and had $3k (or slightly more) in options. I paid $16k (new) for my 99 Contour, which other than the sweet/smooth V6 has no options on it.